REVIEW OF DEEP FOUNDATIONS INTEGRITY TESTING
METHODS AND CASE HISTORIES
Samuel G. Paikowsky1, and Les R. Chernauskas2
ABSTRACT
Deep foundations integrity testing is employed for assessing the soundness of in-place
constructed elements. The increased use of these members in the New England area over the
past 20 years has resulted in an increased demand for quality control testing.
A basic review of small and high strain integrity testing techniques used for deep
foundations along with local case histories is presented. The issue of the required number of
tests is addressed in light of the recent recommendations developed for the Deep Foundations
section of the LRFD specifications. Various applications including pile length determination,
integrity of pressure injected footings and piles during driving are included. Previous
comparative studies are referenced and strengths and weaknesses are highlighted.
The integrity tests are shown as a useful and important tool. This is especially true when
a match exists between the implemented technique, foundation type, the user expertise and the
owners’ expectations. Solid engineering judgment, analysis and decision making enhances the
ability to utilize the test results and hence their usefulness and importance.
1.0
INTRODUCTION
Integrity testing is the process in which the soundness of the inspected object can be
determined. Integrity testing of deep foundations has become common over the past 20 years
due to the combination of construction requirements and technological advances. Growth in the
use of in-place constructed foundations (e.g. drilled shafts) along with higher design loads and
increased legal activities are the main motivation behind the need for integrity testing. Advances
in the areas of instrumentation, data acquisition, and signal processing accompanied the
increased power of personal computers. These advances enhanced the capabilities and reduced
the cost of developing methods for integrity evaluation of foundations.
Deep foundations integrity testing mostly applies to foundations constructed from
concrete/grout, such as drilled shafts, drilled mini piles, pressure-injected footings, and precast
concrete piles. The testing is required for quality control during construction to detect flaws in
the pile (e.g. necking, cracking, void, poor quality material, etc.). Such defects are applicable to
cast (or injected) in place concrete piles, and to a lesser extent to precast concrete piles. In some
cases, the determination of the foundation length is required. Integrity testing can then be
performed on any deep foundation type, (including timber and steel piles) with some methods
1
Professor, Geotechnical Engineering Research Laboratory, Department of Civil and Environmental Engineering,
University of Massachusetts, Lowell, 1 University Ave., Lowell, MA 01854, and a Principal at GTR.
2
Project Manager, Geosciences Testing and Research, Inc. - GTR, 55 Middlesex St. Suite 225, North- Chelmsford,
MA 01863.
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Methods and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P1
being capable of determining foundation length even when the foundation is not directly
accessible e.g. structure/cap coverage of the pile’s top, (Finno et al., 1995).
This paper combines the material presented by Chernauskas and Paikowsky (1999) and
Paikowsky et al. (2000), Chernauskas and Paikowsky (2000), and Paikowsky (2003).
Determining the integrity of a material can be accomplished by intrusive or non-intrusive
methods. Intrusive methods are more conventional, and include drilling, coring, or penetration
via preinstalled conduits. These methods can include destructive testing (e.g. on core samples)
providing direct information about the condition of the structure under consideration, but may
compromise the structural integrity once testing is completed. Non-intrusive testing can provide
information about the condition of the structure without altering its structural integrity. Integrity
testing by non-intrusive methods is often more cost effective, but requires sophisticated
equipment and specialty training to yield meaningful results.
This paper reviews the basic theory and application of the more common NonDestructive Testing (NDT) methods applied to integrity testing of deep foundations. Both
intrusive and non-intrusive techniques are described and relevant case histories from the New
England area are presented. For additional information and analyses, the reader is directed to
publications by Baker et al. (1992), Holeyman (1992), Rausche et al. (1992) and Vyncke and
Van Nieuwenburg (1987).
2.0
BACKGROUND
Two techniques broadly categorize pile testing: small and high strain testing. High strain
testing is aimed at the pile capacity evaluation with the ability to determine its integrity. Small
strain testing is aimed at investigating the pile integrity alone and is based on two principles
related to the measurement of sound/stress waves by either direct transmission or reflection.
Common direct transmission techniques include: (1) crosshole sonic logging, (2) single hole
sonic logging, and (3) parallel seismic logging. In these methods a sonic pulse is produced with
one transducer (transmitter) and the signal is picked up with another transducer (receiver). The
transducers typically consist of a geophone or accelerometer. The methods differ in the location
of the transducers and the pulse generation method. Common surface reflection techniques
include (1) pulse echo (a.k.a. sonic echo), (2) transient dynamic response (a.k.a. impulse
response), and (3) conventional high strain dynamic testing. In these methods reflections of
waves generated at the top of the pile are measured. As both generated and reflected signals are
measured at the same location, more sophisticated instrumentation (typically accelerometers and
strain gages), data acquisition, and signal processing procedures must be employed. The major
difference among these techniques relates to whether the generated impact pulse propagates
under high strain or low strain conditions.
Other common reflection techniques include the use of high frequency, electromagnetic
pulses such as X-ray, and radar. These methods are more commonly used for subsurface soil
evaluation (e.g. stratification, groundwater, and bedrock) and/or concrete slab mapping (e.g.
rebar, voids, thickness and condition determination).
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3.0
DIRECT TRANSMISSION TECHNIQUES - Crosshole Sonic Logging (CSL)
3.1
Technique
The most common direct transmission integrity testing method is crosshole sonic logging
(CSL or sonic coring in Europe). The method is used to evaluate the condition of the concrete
within cast in place piles (caissons or drilled shafts) and slurry or diaphragm walls. A
piezoelectric transducer is used to generate a signal that propagates as a sound (compression)
wave within the concrete and another transducer is used to detect the signal. Each transducer is
placed into a vertical PVC or steel tube that has been attached to the reinforcement cage and
filled with water prior to the concrete placement. The water acts as a coupling medium between
the transducer and the tube. A typical tube arrangement and testing principles are presented in
Figure 1.
The source and receiver transducers are lowered to the bottom of their respective tubes
and placed such that they are in the same horizontal plane. The emitter transducer generates a
sonic pulse (on the order of 10 pulses per second), which is detected by the receiver in the
adjacent tube. The two transducers are simultaneously raised at a rate of around 1-foot per
second until they reach the top of the drilled shaft. Typically this process is repeated for each
possible tube pair combination (perimeter and diagonals). Figure 1b shows the six tube
combinations that can be tested (logged) using a configuration of 4 tubes within a drilled shaft.
Increased shaft diameter calls for a larger number of tubes, which increases, the number of
combinations and thereby the resolution of the testing zone.
A'
A
Figure 1. Typical CSL Testing Set-Up
Showing (a) Transmitter and Receiver at
Different Depths, and (b) Plan View of the
CSL Tubes with Possible Test Combinations.
defect
A - A'
(b)
Transmitter
Signal
Path
Receiver
(a)
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Methods and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P3
In homogeneous, good quality concrete, the stress/sound wave speed, C, is typically
around 12,000 to 13,000 feet per second and is related to the modulus, E, and unit weight, γ, as
follows:
C=
E • g
(1)
γ
If for any reason the condition of the concrete is compromised, the wave speed will be reduced
relative to that of the “sound” concrete value. Figure 2 presents a typical sonic signal for which
the propagation time between the transducers is measured. The vertical axis is the signal
amplitude (microvolts) and the horizontal axis is the time (microseconds). The point where the
amplitude begins to rapidly fluctuate indicates the arrival time of the signal to the receiver (a.k.a.
threshold time). Since the distance between the two tubes is known, the wave speed of the
concrete between the tubes can be evaluated. The signal arrival times can then be plotted with
depth to generate a log for the particular tube combination as presented in Figure 3. In addition
to the threshold times, the energy of each signal may also be plotted with depth. This
information can be used to compare signals of one zone to another where lower energy and/or
longer arrival times correspond to a compromised quality and/or defect.
Amplitude (volts)
Threshold
Noise
Time (microsec)
Figure 2. CSL Testing Typical Signal
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2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P4
Threshold Time (microsec)
Debonding
Defect
Depth
Threshold
Energy
Defect
Soft Bottom
Energy (volt-microsec)
Figure 3. Presentation of CSL Test Results in the Form of Threshold Time and Energy vs
Depth.
Advantages to this method include the direct assessment of pile integrity and the ability
to position the transducers in different elevations to create more signals, allowing the
development of a tomographic presentation of the investigated zone. The limitations of the
method include detection of defects only when they exist between the tubes. The testing can be
performed only on drilled shafts for which access tubes were installed. Also, the method can
only be used for drilled shafts, as other deep foundations are usually too small or constructed
using different methods that do not lend themselves for accommodating the access tubes.
Debonding between the tubes and concrete is common if testing occurs long after the concrete
placement. Testing in fresh concrete is also difficult as certain zones may cure at a lower rate,
creating difficulties in the interpretation of the threshold time and energy. These zones may
therefore be interpreted as poor quality concrete.
3.2
CSL Case History – Equipment and Testing
3.2.1 The PISA CSL/SSL Testing System
The PISA (Pile Integrity Sonic Analyzer) is a modular system allowing for adoption,
upgrade and incorporation of additional integrity testing technologies. The current integrity
testing options available in the PISA include cross-hole sonic logging (CSL) and single-hole
sonic logging (SSL) using CHUM (Cross-hole Ultra Sonic Module) and sonic echo (a.k.a. small
strain propagation) using the PET (Pile Echo Tester) module. Additional modules are currently
under development.
In addition to its modularity, two advantages of the PISA integrity testing system over other
systems include its software and portability. The PISA is A Windows based system and is also
compatible with Word 2000. The software is updated periodically to incorporate new developments
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2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P5
and algorithms that make data collection, interpretation, and report preparation easier and efficient.
The PISA is lightweight (only 42.3 N (9-1/2 lb)) and self powered, hence can be easily carried around
from shaft to shaft or site to site. This feature is also beneficial for air travel. The system can be also
used as a standard laptop, saving the cost and space required for an additional personal computer (PC)
when using a dedicated CSL testing system.
Figures 4 through 8 present photographs of the PISA system, including computer and
sensors. Figure 5 presents the layout of the pile screen, where one can enter the pile information
and select the tube orientation/locations. Selection of the desired tube combinations is
accomplished by drawing a line between any two tubes. Figure 6 presents the data collection
screen, where real-time graphical presentation of the concrete integrity is provided during
testing. If a suspect zone is detected in this stage and the tomography option is enabled, the
probes are lowered and raised relative to each other around the suspect zone, to further
investigate and delineate the area. The signals can be examined and adjusted by manually
picking the points or using preset algorithms to automatically determine the first arrival time
(FAT) as shown in Figure 7. Figure 8 represents the typical graphical output for time and energy
plots.
Figure 4. The Pile Integrity Sonic Analyzer (PISA) System with CHUM and PET Modules
1 The PISA Laptop PC
2 The PISA Module
3 Wheels for Cables and Depth Encoders Mounted on the CSL Access Tubes
4 Transducers and Cables
5 PET Transducer
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Figure 5. Layout of the “Pile” Screen
Figure 7. First Arrival Signal Identification Screen
Figure 6. Data Collection Screen
Figure 8. Typical Graphical Output for Time and Energy Plots
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Methods and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P7
3.2.2 Recent Advances in CSL Testing
Following recent technological advances, a new concept in NDT equipment has emerged
(Amir and Amir, 1998a,b). The use of generic laptop PC based systems and modular equipment
components seem to be taking the place of the older dedicated systems. Naturally, the new
concept allows small size, lighter, independent equipment with broader NDT applications. Such
equipment has the advantage of employing common operating systems (e.g. MS Windows),
conforming to other requirements (i.e. graphics presentations and word processing). In addition,
such systems can easily utilize updated algorithms, for example, real time on screen tomographic
presentation. The first experience in the USA of such equipment was the PISA (described in the
previous section) and a relevant case history is discussed in the next section.
A newer version of that system recently developed and distributed is presented in Figure
9. The USB (Universal Serial Bus) based CHUM is a generic box connected to any PC using the
USB port (shown in Figure 9 connected to a tablet PC), hence allowing a flexible yet affordable
system. This kind of device will most likely result in a new generation of NDT equipment that is
more affordable and better suited for versatile testing demands, advanced analyses, and field
application.
Dimensions
250mm x 210mm x 74mm
Figure 9. USB based CHUM Connected to a Tablet PC.
3.2.3 Testing
CSL testing using the PISA were carried out by Geosciences Testing and Research, Inc.
(GTR) of N. Chelmsford, MA, on over 100 drilled shafts installed for the support of a major
roadway interchange in Boston, Massachusetts. The shafts ranged between 2.1m (7 ft) and 2.7 m
(9 ft) in diameter and tapered to 1.2 m (4 ft) to 1.5 m (5 ft) in diameter over the lower portion (15
m (50 ft) to 30 m (100 feet)). The total lengths varied between 36 m (120 ft) and 67 m (220 ft)
below ground surface. Various penetrations into rock were required depending on the loading
conditions. The shafts were constructed using temporary steel casing to the top of the clay and
slurry throughout the remainder of the drilling and concrete placement process. The concrete
was placed using a tremie process.
Eight schedule 80 PVC CSL access tubes were attached to the reinforcement cage and
placed within the shaft prior to the placement of the concrete. The tubes were filled with water
prior to placement in the shaft. CSL testing was performed primarily along the four diagonal
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2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P8
tube combinations and four edge tube combinations as shown in Figure 10. Additional testing
was performed as needed depending on the results.
1
CALLED
NORTH
8
2
7
3
6
4
5
Figure 10. CSL Tube Layout
The CSL testing indicated significant anomalies in four of the first group of shafts
installed for phase 1. Typical time and energy plots for the shafts (designated as "1 through 4")
showing the various anomalies, are presented in Figures 11 through 14, respectively. The
presented data illustrate two soft bottoms (shafts 1 and 2) and two problems in the upper 12.2 m
(40 ft) (shafts 3 and 4). Table 1 summarizes the results of the repeated CSL testing performed
for all four shafts. Following the testing and outline of anticipated problematic zones, a coring
program was undertaken to verify the identified anomalous zones. For shafts 1 and 2, one
10.16-cm (4-in) core was drilled down the center of each shaft into the underlying bedrock. For
shafts 3 and 4, multiple cores were taken between 6 m (20 ft) and 9 m (30 ft) as shown in
Figures 15 and 16, respectively. The lateral extent of the suspect zones as identified by the CSL
testing is presented in the two figures.
The concrete core samples retrieved from the suspect zones in shafts 1 (lower 6.1 m (20
ft)) and 2 (lower 2.4 m (8 ft)) were completely raveled, segregated, and disintegrated suggesting
major discontinuity in the lower section of the shaft. The beginning of the defective concrete
obtained from the coring coincided closely with the suspect zone identified during the CSL
testing. These two shafts had gone through extensive repair procedures to transfer the loads
through the defective zone to the underlying rock below.
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Methods and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P9
Table 1 Cross-Hole Sonic Logging Test Summary
Shaft
Shaft 1
1
Concrete
Tested
Age when
Length
Tube Stickup
Tested
(feet)
(feet)
Average 2
Condition
3
14 days
197 to 205
10 to 15
Edge Tube combinations: showed weak signal or loss of signal for a 2 to 3' zone in each profile within lower 20
to 30' of shaft (soft bottom).
Diagonal Tube combinations: showed frequent weak signal or loss of signal in each profile over lower 20 to 30'
of shaft (soft bottom).
20 days
197 to 204
10 to 15
Diagonal Tube combinations: showed slight improvement over lower 20' of shaft, but still indicated soft bottom
in each profile.
30 days
199 to 203
10 to 15
Diagonal Tube combinations: showed further improvement over lower 20' of shaft, but still indicated soft
bottom in each profile.
9 days
109 to 116
15 to 20
All tested tube combinations indicate soft bottom conditions (lower 2 to 6 feet) except tube combination 1-3.
30 days
111 to 114
15 to 20
All tested tube combinations indicate soft bottom conditions (lower 6 to 8 feet).
4 days
177 to 178
15 to 20
Anomaly 25 to 35 feet below top of tubes in most profiles.
12 days
175 to 178
15 to 20
Anomaly 25 to 30 feet below top of tubes in northwestern quadrant of shaft.
18 days
130 to 135
15 to 20
Anomaly 20 to 40 feet below top of tubes in most profiles.
30 days
137 to 139
15 to 20
Anomaly 20 to 40 feet below top of tubes in most profiles.
44 days
133 to 136
15 to 20
Anomaly 35 to 45 feet below top of tubes in northwestern quadrant of shaft.
87 days
126 to 133
15 to 20
Anomaly 35 to 45 feet below top of tubes in northwestern quadrant of shaft.
Shaft 2
Shaft 3
Shaft 4
Notes.
1. The tested length represents the average length of tube accessible during CSL testing, including the stickup above the top of the concrete. The tested length may vary for several
reasons, including different tube stickups, slack in cables, blockage in tubes, and/or slippage between depth wheel and cable.
Notes:
2. The average stickup was not the same every time, but varied between the numbers indicated for these tests.
3. The lengths and depths are referenced from the top of the tubes. Subtract the tube stickup to determine the depth in the drilled shaft.
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Methods and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P10
0
25
Arrival Times
Relative Energy
50
Depth (ft)
75
100
125
150
175
200
0
0.2
0.4
0.6
0.8
Time (ms) and Energy (relative scale)
1
Figure 11. Shaft 1 - Time and Energy with Depth
0
20
Arrival Times
Relative Energy
Depth (ft)
40
60
80
100
120
0.0
0.2
0.4
0.6
0.8
Time (ms) and Energy (relative scale)
1.0
Figure 12. Shaft 2 – Time and Energy with Depth
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P11
0
Arrival Times
Relative Energy
20
40
Depth (ft)
60
80
100
120
140
160
180
0.0
0.2
0.4
0.6
0.8
Time (ms) and Energy (relative scale)
1.0
Figure 13. Shaft 3 – Time and Energy with Depth
0
Arrival Times
Relative Energy
20
Depth (ft)
40
60
80
100
120
0.0
0.2
0.4
0.6
0.8
Time (ms) and Energy (relative scale)
1.0
Figure 14. Shaft 4 – Time and Energy with Depth
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P12
Note: Core locations are approximate
1
1
CALLED
NORTH
CALLED
NORTH
8
Note: Core locations are approximate
8
2
4" Core
2" Core 3
4" Core
2
4" Core
2" Core 1 2" Core 2
2" Core 1
7
2" Core 2
7
3
3
2" Core 3
Legend
5
Tube stickup 15 to 20'
Weak to no signal from 25 to 30 feet below tubes
Good signal
Estimated maximum possible area of suspect zone
Realistic area of suspect zone
(around 9 square feet in area)
Figure 15. CSL Summary - Shaft 3
4
6
4
6
5
Legend
Tube stickup 15 to 20'
Weak to no signal from 30 to 45 feet below tubes
Good signal
Estimated maximum possible area of suspect zone
Realistic area of suspect zone
(around 18 square feet in area)
Figure 16. CSL Summary - Shaft 4
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P13
The concrete core samples retrieved from the suspect zones in shafts 3 (upper 1.5 to 3 m
(5 to10 ft)) and 4 (upper 6.1 to 9.1 m (20 to 30 ft)) indicated mixed results. Moderate to severe
segregation was observed in some of the cores for shaft 3. In one core, complete disintegration
was observed between 1.5 to 3 m (5 to 10 ft). Some slight to moderate segregation was observed
in the core samples retrieved from shaft 4. Compressive strength, Elastic Modulus, and unit
weight testing were performed on selected samples from the cores obtained from these two
shafts. The compressive strength testing indicated high variability in the values for both shafts.
Between 1.5 to 3 m (5 to 10 ft) below the top of shaft 3, the compressive strength was
considerably lower (some values were much less than the required 28 day strength) than the
areas above and below. Lower compressive strength values were also observed for shaft 4
specimens results below 6.1 m (20 ft). The concrete specimens tested from this shaft exhibited a
wide variability in the compressive strength. These depths correspond to the anomalous zones
identified during the PISA CSL testing, suggesting compromised concrete condition. Shafts 3
and 4 were re-evaluated for their structural load carrying ability and subsequently were found to
be adequate inspite of the compromised zone.
4.0
ADDITIONAL DIRECT TRANSMISSION TECHNIQUES
4.1
Singlehole Sonic Logging (SSL)
Singlehole sonic logging (SSL) is a variation of the direct transmission CSL method in
which the source and receiver are placed in the same tube and the signal travels in a vertical
direction (refer to Figure 17). The method is limited to defects adjacent to the tube and is
usually used only when a drilled shaft requires integrity assessment after construction. Due to
high coring costs, typically, a single hole is advanced (often down the middle) to the bottom of
the shaft or slightly below the depth where a defect is anticipated. It may also be desirable to
perform SSL during CSL testing to isolate the location of a defect at a certain depth (i.e.
distinguishing whether the defect identified using CSL is adjacent to the tube or in between the
tubes). Brettman and Frank (1996) describe a comparison between CSL and SSL tests.
4.2
Parallel Seismic Logging
Parallel seismic logging is another direct transmission integrity testing variation of the
CSL test. The method is primarily performed for the assessment of the length of older
foundations. Although large voids or bulges can be identified along the deep foundation edge, it
is not typically used for identifying defects. Figure 18 presents the procedure in which a boring
is drilled in the ground adjacent to the existing deep foundation (usually within 2 to 3 feet of the
deep foundation edge). The drilled hole is advanced well beyond the estimated tip elevation to
ensure that the entire deep foundation profile can be logged. A capped PVC tube is placed
within the drilled hole and surrounded with bentonite slurry/grout that bonds the tube to the edge
of the boring.
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P14
Depth
Threshold Time
Defect
Signal
Path
Receiver
Transmitter
(a)
Signal
Path
Receiver
Figure 17. Typical SSL Test Set-up Showing the
Transmitter and Receiver Placed at Different Depths.
(b)
Figure 18. A typical Parallel Seismic Testing Arrangement
Showing a) Instrumented Hammer and Receiver at Several
Depths, and b) Threshold Time versus Depth.
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
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A receiver transducer is placed at the bottom of the water filled tube and pulled upwards
in intervals of around 1 or 2 feet. At each depth interval, the foundation top is struck with an
instrumented hammer that sends a pulse down the pile and the soil, to be detected by the
receiver. A typical profile of the signal arrival time with depth can be logged as shown in Figure
18b. A change in the rate of signal arrival time signifies either a large defect or the end of the
pile.
The most attractive feature of this technique is that any deep foundation type can be
tested as long as the drilled hole is close to the foundation. Due to the higher cost associated
with drilling, the technique is used to identify foundation depth only when other methods (e.g.
PEM) fail.
5.0
SURFACE REFLECTION TECHINQUES
5.1
Pulse Echo Method (PEM)
The Pulse Echo Method (PEM, also known as the sonic echo method) is a surface
reflection integrity testing technique. A high frequency accelerometer is attached to the pile top
using a mild bonding agent such as petro wax or petroleum jelly. A lightweight hand held
hammer (1 to 3 pounds) is used to strike the pile top and generate a small strain stress wave. The
strains associated with the propagating stress wave are in the order of 1µε. Figure 19 presents a
typical PEM integrity testing setup. The hammer is usually constructed from plastic to minimize
the extraneous high frequencies generated by steel. The accelerometer attached to the pile top
measures the acceleration at impact and reflections arriving to the surface from within the pile.
This analog acceleration signal is recorded, digitized, and integrated (using a computer) to create
a velocity record.
Figure 19. A Typical PEM
Integrity Test Set-up.
Defect
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P16
Velocity (feet/sec)
The velocity record indicates the speed at which the pile material (at the point of
measurement) moves due to the impact and reflected stress waves created by the hammer. A
typical velocity record is shown in Figure 20a. This record can be further processed using
algorithms that enhance the signal through filtering, shifting, pivoting and magnification. This
manipulation allows enhancement of the velocity signal for weak toe reflections, reduce the
effect of unwanted noise and drifts, thereby aiding in the interpretation of the pile response.
2L/C
2L/C
Force (lbs)
Time (millisec)
(a)
Time (millisec)
(b)
Figure 20. Typical PEM a) Velocity and b) Force Records.
Changes in the pile cross-section, concrete density, and/or soil resistance affect the
impedance in the direction of the traveling wave and create reflections of the stress wave that
propagate back towards the pile top. These reflected stress waves can return in compression or
tension, depending on the type of impedance change. The pile properties that define impedance,
Z, are the speed at which the stress wave propagates, C, the elastic modulus, E, and the cross
sectional area, A expressed as:
E • A
Z =
(2)
C
Figure 21 illustrates the relationship between the variations in the pile impedance, the
traveling wave and the reflections recorded at the surface. A reflected tension wave indicates a
decrease in impedance. Conversely, a reflected compression wave indicates an increase in
impedance. Combinations of these impedance changes can create complex reflections at the pile
top. By inspecting the velocity record for these changes, the approximate location of the
impedance change can be determined. At a time of 2L/C, where L is the pile length, the pile toe
response can be identified by observing a reflected tension wave due to softer soil at the tip
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P17
Z1
Z1
Z2
Z1
Time (msec)
Depth (feet)
Velocity (ft/sec)
(signal is in opposite direction of impact pulse analogous to free end conditions) or a reflected
compression wave due to denser soil at the pile tip (signal is in same direction as impact pulse
analogous to a fixed end condition). These reflections are subsequently discussed for the high
strain testing. One of the most difficult tasks in the interpretation of the velocity record is
distinguishing between the velocity reflectors due to pile defects (e.g. crack, neck, void, poor and
velocity reflectors due to soil resistance. Detailed quantification of defects is difficult, if not
impossible, since the interpretation is based on reflected waves and also relies on an assumed
wave speed. The most reliable way to use the method is by comparing the response from a large
number of piles at the same site. Piles that indicate a response that is different from the majority
are further investigated.
Z2
Z1
Figure 21. Wave Propagation and Reflections versus Time and Depth.
The PEM testing is simple and quick and hence can often be performed on all the piles at
a site. PEM testing can be carried out on various deep foundation types and materials. Under
certain conditions, the PEM can be performed on piles that have been covered by a cap or grade
beam structure. The small strain PEM technique is generally effective to a depth of 20 to 30 pile
diameters, depending on the magnitude and distribution of the frictional soil resistance.
5.2
Transient Dynamic Response (TDR) Method
The Transient Dynamic Response (TDR) method (a.k.a. the impulse response method) is
based on the PEM except that an instrumented hammer is used to generate the impact pulse. An
accelerometer mounted in the hammer, or a force transducer built in an impulse hammer, allows
one to determine the impact force (using the hammer’s mass) in addition to the velocity records
obtained in the PEM (see Figure 20). Since a force transducer is not attached to the pile, only
the impact force is recorded. The force and velocity records can be converted from the time
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P18
Mobility - V/F (ft-sec/lbs)
domain to the frequency domain using a Fast Fourier Transform (FFT). The ratio of the velocity
spectrum over the force spectrum yields the mobility spectrum (V/F in the frequency domain,
presented in Figure 22), providing an indication of the pile’s velocity response due to the
induced excitation force.
The TDR method allows additional insight compared to the PEM interpretation
technique. Certain dominant frequencies can be identified and correlated to pile length and
distance to variations either in the pile impedance or in the soil. In addition, the low frequency
components (less than 100 Hz) can provide an indication of the dynamic stiffness of the pile.
Although low strain methods allow an estimate of the static pile behavior, they cannot accurately
determine the pile bearing capacity. In contrast to dynamic measurements during driving or
static load test to failure, these methods do not fully mobilize the pile’s resistance.
df
df
df
Frequency (Hz)
Figure 22. A Mobility Spectrum (V/F versus Frequency) Using Records Obtained by the TDR
Method.
6.0
PEM/TDR Case History I - Pressure Injected Footings
Approximately 600 Pressure Injected Footings (PIFs) were installed as part of the
foundation system for a large entertainment complex in Worcester, MA. Two PIFs were visually
observed to contain poor quality, low strength concrete reduced to a “putty like” consistency
near the pile tops. The upper few feet of these PIFs were cut off to remove the material and
assess the extent of the defective zone.
Ten PIFs, including the visually observed defective piles, were selected for PEM/TDR
integrity testing in order to assess the concrete quality in the shafts. The tests were carried out
by Geosciences Testing and Research, Inc. (GTR) of N. Chelmsford, MA. The shafts consisted
of corrugated metal shells filled with cast in place concrete. Reinforcement steel was installed
within the upper 5.5 feet to allow for connection to the piles cap. The subsurface profile in the
vicinity of the test area includes 5 to 20 feet of granular fill over dense sand and gravel. The PIF
bulbs were formed in this denser stratum.
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P19
Figure 23a presents the velocity record with pile length for a ‘sound’ PIF. The signal
indicates a decrease in the velocity around 24 feet, signifying a compression wave reflection due
to the transformation from the shaft to the bulb, corresponding to an increase in the impedance.
The velocity increases sharply at around 26 feet, due to a tension reflection from the bottom of
the bulb (where the impedance decreases when transforming from concrete bulb to the
surrounding sand). The mobility spectrum for this PIF is presented in Figure 24a where peaks
about 256 Hz apart appear between approximately 400 and 1600 Hz. This change in frequency,
∆f, corresponds to a length of around 25 feet, based on the following relationship between time
and frequency:
1
t=
(3)
∆f
and time and distance (considering reflection):
L=C⋅t/2
(4)
The PIF shaft length was reported to be 23.7 feet, which closely agrees with the abovedetermined length considering the accuracy of the construction method and the testing
procedure.
Figure 23b presents the velocity record with pile length for a PIF that was found to have
a major defect. The velocity increases sharply at around 7 feet due to a discontinuity associated
with a large reduction in the impedance. In fact, the low magnitude stress wave could not pass
through this defect and the reflections are repeated every 7 feet with the signal dampened with
time. Even though the PIF shaft was reported to be 23.7 feet, the length indicated by the test,
was around 7 feet, since the defect occupied almost the entire cross section. The mobility
spectrum in Figure 24b looks significantly different from that of a “sound” pile presented in
Figure 24a. In this case, the change in frequency is around 928 Hz, which corresponds to a
length of 7 feet.
The soil around the “compromised” PIF was excavated to a depth of 10 feet. The
corrugated shell was torched off the shaft around 8 feet below the top of the pile. When the shell
was removed, the PIF fell over, due to a complete break in cross section around 7.5 feet.
Another PIF evaluated by the PEM/TDR to have a defect around 5 feet was also excavated.
After the corrugated shell was removed, a large volume of water and “putty like” concrete fell
out of the shell. An approximate 20% reduction in cross section was observed at a depth of 4 to
5 feet below the top of the PIF. As a result of the integrity testing and subsequent verification in
the field, a reduced cross sectional area was used to reassess the load carrying ability of the
foundations.
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P20
Figure 23. PEM Velocity Records versus Time for a) a Sound Pile and b) a Defective Pile (for
PIFs).
Figure 24. TDR Mobility versus Frequency Responses for a) a Sound Pile and b) a Defective
Pile (for PIFs).
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P21
7.0
PEM/TDR Case History II – Precast Concrete Driven Piles
Damage in driven piles can be detected while monitoring the pile capacity using high
strain dynamic measurements (subsequently presented). These tests are traditionally carried out
on a small number of piles even when compared to typical concrete pile breakage during
installation (say 5 to 7%). Damage during driving or site work following the installation can
result in piles with questionable integrity.
Figures 25a & b present PEM testing results on 16 inch square (about 90 feet long)
concrete piles driven for the support of multistory buildings in Cambridge, Massachusetts. The
tests were carried out by Geosciences Testing and Research, Inc. (GTR) of N. Chelmsford, MA.
A repetitive increased velocity reflection at times corresponding to a distance of about 15 to 20
feet is presented in Figure 25a. The repetitive reflection indicates damage extensive enough to
prevent the signal from propagating any deeper than the indicated depth. No evidence is
however provided by the test as to the compression load bearing capability of the pile. Figure
25b presents the results obtained from a nearby sound pile for which the propagating signal
responded to the variation in the soil type (sand layer at about 25 to 30 feet and a till layer at
about 60 to 70 feet). The tip response was magnified due to the small energy used in the PET
testing. As a result, the techniques’ effectiveness at such depths is questionable.
The usefulness of the method in time and cost savings was certainly a big advantage,
allowing the identification of a large number of defective piles in a short period of time. The
limitations regarding the nature of the damage and the structural ramifications need to be
recognized as well.
Figure 25a. PEM Velocity
Records versus Time for a) a
Defective Pile
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P22
Figure 25b. PEM Velocity
Records versus Time for b) a
Sound Pile (Precast Concrete
Driven Piles).
8.0
HIGH STRAIN INTEGRITY TESTING DURING PILE DRIVING OR DROP
WEIGHT TESTING
8.1
Technique
Dynamic pile testing is commonly employed for evaluating the drivability and capacity
of driven piles. The same method is also used to assess the capacity of cast in place shafts.
When a ram strikes the pile head, it initiates a large strain wave that propagates down the pile as
illustrated in Figure 26. External soil resistance or changes in the pile’s impedance (due to
variations in the pile’s material or geometry) causes reflection waves that are recorded at the
surface in what is in principle a surface reflection technique similarly to PEM/TDR methods
using low strain waves. Typical dynamic pile testing instrumentation consists of two
accelerometers and two strain transducers attached on opposite sides of the pile close to the pile
top. Knowing the material properties and pile geometry at the point of measurement, the strain
is converted to force, while the acceleration is integrated with time to produce a velocity record.
These force and velocity records can be used to evaluate the pile’s integrity. As long as there is
no change in the pile impedance or external forces (friction) are not activated, the force and
velocity remain proportional. Reflections from the tip can be reviewed in light of two classical
boundary conditions (see for example Timosheko and Goodyear, 1934). Free end (analogous to
easy driving through soft clay) calls for zero stress and no velocity restrictions at the tip,
resulting in a compression wave returning as a tension wave and velocity increase (theoretically
doubling). Figure 27 presents such reflection from a 48-inch diameter pipe pile driven offshore
during initial penetration of about 3 feet. The downward velocity and compression stress
returned from the tip as a tension wave and an increased downward velocity. Fixed end
conditions (analogous to hard driving against bedrock) calls for zero velocity and no stress
restrictions at the tip, resulting in a compression wave being reflected with a greater magnitude
than the incident wave and the tip velocity at about zero.
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P23
RAM
Strain Gage
traveling impact
(incident) waves
Accelerometer
Figure 26. A Typical
Dynamic Test Set-up.
returning reflection waves
Defect
Figure 27. Measured Force and Velocity (Times the Pile Impedance) at the Pile Top versus
Time during Initial Penetration (Paikowsky, 1982).
If a pile contains a defect or is damaged during driving, the wave reflecting from the zone
of decreased impedance is comparable to “free end conditions”. These reflections would arrive
to the measuring transducers before the reflections associated with the pile’s tip as the damaged
zone is at a point along the pile between the top and the tip. The detection of damage during
driving is routine and usually is associated with tension cracking of concrete piles. Other
structural damage (e.g. splice breakage) can also be identified as presented in the case history
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P24
below. The advantage of the high stress wave propagation testing over the small strain integrity
testing is its ability to quantify the structural significance of the discontinuity. While a small
strain wave would indicate a complete discontinuity for any size crack across the pile, the high
strain stress wave would pass through these discontinuities enabling the transformation of
compression forces, therefore indicating the adequacy of the structural member.
8.2
Case History – Driven Pile
Several hundred H piles were installed for the support of an elevated walkway in the
Boston area. Dynamic pile testing was specified for capacity monitoring and the driving
operation progressed routinely. One of the inspected piles exhibited clear damage progression
during driving. Figures 28a and 28b present the force and velocity (multiplied by the pile’s
impedance) signals at the pile top shortly before and after damage detection. Since the early
damage identification was dismissed, driving continued and the dynamic records for the
subsequent blows are presented in figures 28c, d, and e. A clear velocity increase accompanied
by a force decrease attests to the development of the damage. The records of Figure 28e suggest
that the pile essentially “ends” at mid-point, indicating a complete detachment between the upper
and lower pile sections. The identified damage is associated with a full penetration weld splice
that apparently disintegrated during driving. When the pile was pulled out of the ground only
the upper section was extracted with severe deformations at the weld connection.
8.3
Case History – CFA Shafts
Paikowsky et al. (2004) report on drop weight testing of Continuous Flight Auger (CFA)
and Bentonite slurry construction shafts. The shafts were tested as pre-construction evaluation
of possible foundation solutions for a large multi-story expansion of a pharmaceutical
manufacturing complex in Haifa Bay, Israel. The three CFA shafts were 70cm in diameter and
25.0m long. The tests were conducted using a modular drop weight system shown in Figure 29.
When analyzing the dynamic test results using the signal matching technique (CAPWAP) and
the reported shaft dimensions, it became apparent that the lower parts of all three shafts exhibit
substantially reduced impedance as depicted by the impedance distribution (solid line) in Figure
30. Often in drop weight testing, the produced stress wave is either not sharp enough or the
energy is not high enough to mobilize the shaft’s tip, and hence its clear detection (as in the
presented tests). As such, the reported constructed length of 25.0m was used in the analysis.
The results depicted in Figure 30 can be interpreted as either the quality of the shaft in the lower
5m was compromised, (compared with an expected impedance of 3,765kN/m/s) or the shaft was
not constructed to the planned depth of 25m. Further analysis was carried out for possibly
shorter piles resulting with the impedance distributions depicted by dashed lines in Figure 30.
These distributions reflect shaft lengths varying between 22.0 to 23.0m and suggest reasonably
well-constructed shafts, but shorter than anticipated. This result was later confirmed to be the
correct answer.
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P25
Figure 28. Force and Velocity Records Obtained during the Driving of a Steel H-Pile a) Shortly
before Detecting Damage, b) Showing Initial Damage, c) as the Damage Develops, d) as the
Damage Progressed and e) Upon Complete Discontinuity.
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P26
GUIDES
TO CRANE
TRIPPING
MECHANISM
LIFTING
DEVICE
CONNECTING
RODS
MODULAR
MASSES
STRIKER PLATE
ADJUSTABLE
DROP HEIGHT
PILE ENCLOSER
PILE
CUSHION
PDA
INSTRUMENTATION
PILE
GROUND SURFACE
(a) schematic
Figure 29. Israeli Drop Weight System (after GTR, 1997).
Impedance (MN/m/s)
Depth Below Grade (m)
0
2
4
6
8
10
0
2
4
6
8
10
0
0
0
0
5
5
5
10
10
10
2
4
6
8
10
C3
C2
C1
15
15
15
20
20
20
25
25
25
Figure 30. Variation of Shaft Impedance with Depth for the CFA (C1 – C3) Constructed Shafts
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P27
9.0
NUMBER OF TESTS
A study aimed at the development of new AASHTO Load and Resistance Factor Design
(LRFD) specifications for deep foundations was recently completed (Paikowsky, 2002). One of
the issues addressed in the study was the number of tests recommended/required either for
dynamic tests on driven piles, or small strain tests for drilled shafts. The two have a different
logic that supports the tests and, hence, lead to a different probabilistic approach in their
solution. While the first looks for the number of tested piles required to verify capacity
evaluation, the second looks for the number of tests to guarantee a limited tolerable number of
defected shafts.
The statistical approach for both is presented by Paikowsky (2002), and was carried out
in collaboration with Professor Gregory Baecher (see Paikowsky, 2002). For the drilled shafts
the analysis requires to define the maximum fraction of shafts with a major defect that the owner
is willing to tolerate in a large set of shafts. A major defect was defined as any defect that
significantly compromises the ability of the shaft to carry the assigned loads. A sample
calculation was carried out assigning this value for 5% along with: (1) the “owner’s” risk of
incorrectly accepting a defected shaft as a solid shaft to be 10%, and (2) the “contractor’s” risk
of rejecting a set of solid shafts as defected being 10%. This calculation suggested that for 100
shafts in a sample, one needs to test 80 shafts in order to comply by the above requirements,
which are not seen as stringent requirements. It was concluded, therefore, that in order to
statistically assure very low rates of major defects within a set of drilled shafts, a very high
proportion of the shafts must be tested. Thus, it became reasonable to require 100% of drilled
shafts to be post construction tested for major defects.
Reasonably such tests can be a combination of direct transmission (e.g. CSL) for a
limited number of shafts, e.g. during initial stage of construction and at several locations based
on site and/or construction variations, complemented by the more cost effective surface
reflection tests (e.g. PEM) on all other shafts.
10.0
DISCUSSION
A large variety of non-destructive, intrusive and non-intrusive deep foundation integrity
testing methods and case histories are presented. The methods’ strengths and limitations are
related to their effectiveness, time (in preparation, testing and interpretation) and associated cost.
In general, the direct transmission methods necessitate considerable preparation and can provide
higher accuracy in the zone bounded by the penetrating sleeves. Surface reflection techniques
require only minimal preparation but are limited in their zone of meaningful operation and
accuracy. The selected testing method needs to reflect the anticipated result and the associated
line of action. It is with this approach in mind that one requires reviewing comparative studies
of known embedded defects (e.g. Baker et al., 1992 and Smits, 1996).
The ability of a method to detect a certain defect should be examined in light of the
defect’s influence on the foundation serviceability. This course leads to the selection of an
integrity testing method based on the expected outcome. For example, the possible detailed data
provided by the direct transmission methods should not result in a caisson rejection just because
certain zones suggest a lower quality of concrete. Such decisions need to be associated with the
design loads and the load bearing assessment of the tested caisson. The surface reflection
methods on the other hand allow extensive testing with the expectations that detailed
Paikowsky and Chernauskas, Review of Deep Foundations Integrity Testing Techniques and Case Histories
2003 BSCES-GEO-INSTITUTE DEEP FOUNDATION SEMINAR P28
investigations are carried out on the suspected caissons only. Choices, therefore, should be made
regarding quantity and quality. Testing of many piles with the ability to detect major defects
(where possibly undetected defects are not expected to compromise the pile’s load carrying
ability) vs. detailed studies of a smaller number of piles or a combination of the two methods can
be performed. Statistical evaluation of risk associated with major defects supports the logic of
testing each and every cast-in-place deep foundation.
11.0
CONCLUSIONS
The reviewed methods and the presented case histories demonstrate that deep
foundations integrity testing is useful and has significant importance. The PEM/TDR and CSL
techniques are routinely used to assess the quality and condition of cast in place and driven piles.
Conventional dynamic testing is effective in evaluating pile integrity during driving or striking if
in place constructed deep foundations. In some cases the results of the integrity testing were
used to reject the piles where in other cases they were used to re-evaluate or redesign the piles.
Frequently, the integrity testing is used to confirm anticipated defects in the piles. When using
an adequate testing method along with engineering judgment, integrity testing of deep
foundations can be employed as an important tool with sound economical justification.
ACKNOWLEDGMENT
Geosciences Testing and Research, Inc. (GTR) of N. Chelmsford, MA carried out and
interpreted all the described tests. The cooperation of the contractors, consultants and owners (in
particular MHD) associated with the described projects is appreciated. The case history
describing the CSL testing method was carried out using the PISA, Pile Integrity Sonic Analyzer
manufactured by Pile Test Com Ltd. Israel. The case histories described in the section related to
the high strain integrity testing was carried out using the PAK 586 pile driving analyzer
manufactured by Pile Dynamics Inc. of Cleveland, Ohio. The material presented in this paper
was compiled mostly from the following publications: Paikowsky and Chernauskas (1999,
2000), Paikowsky (2002), and Paikowsky et al. (2004). The authors acknowledge the assistance
of Mary Canniff in putting together the manuscript.
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